The present invention relates to an image display apparatus used in a liquid crystal television set.
Known examples of a liquid crystal television set are the active matrix type and the multiplex type. Since the duty ratio of the active matrix type liquid crystal television set is substantially 100%, there is less degradation of image quality if the number of pixels is increased. On the other hand, it is difficult to manufacture this type of television set, which results in a poor yield. Examples of the active matrix type liquid crystal television are disclosed in U.S. Pat. Nos. 4,393,380 and 4,582,395.
In contrast, the multiplex type liquid crystal television set is relatively easy to manufacture and thus is well-suited for mass production. However, since this type of set uses an A/D converter, then in order to increase the number of pixels, it is necessary to provide an A/D converter which can operate at high speed.
As is shown in FIG. 1, a conventional image display apparatus used in a multiplex type liquid crystal television set comprises A/D converter 1 for converting luminance signal Y, supplied from an image amplifier (not shown), into, for example, four bit data D1 through D4, segment driver 2 for driving segment electrodes of liquid crystal display panel 5, shown in FIG. 2, in accordance with data D1 through D4, common driver 3 for sequentially driving the common electrodes of panel 5, and timing controller 4 for supplying a variety of timing signals to A/D converter 1 and drivers 2 and 3. In panel 5 shown in FIG. 2, all the segment electrodes are extracted or accessed from one side, e.g., the upper side of the panel. Note that FIG. 1 shows a case wherein panel 5 consisting of 112.times.288 dots as shown in FIG. 2 is to be driven.
According to the above conventional method, assuming that an effective period of one scanning line is T (sec) and the number of horizontal pixels is n, a sampling frequency of A/D converter 1 and a data transfer frequency fs of driver 2 are represented by: EQU fs=n/T (Hz)
Therefore, if the number of pixels is increased to improve horizontal resulution, frequency fs is increased in proportion thereto. For this reason, the following problems arise:
(1) A/D converter 1, which can operate at high speed, and driver 2 are expensive and power-consuming.
(2) As the speed of A/D converter 1 is increased, the harmonic component of the digital signal has a high frequency, and thereby interferes with the television receiver.
As shown in FIG. 3, an image display apparatus in a conventional liquid crystal color television set comprises A/D converters 11, 13, and 15 for respectively A/D-converting the primary color signals of R, G, and B supplied from a chroma circuit (not shown), segment drivers 12, 14, and 16 for driving the segment electrodes of color liquid crystal panel 19, shown in FIG. 4, in accordance with output signals from A/D converters 11, 13, and 15, respectively common driver 17 for sequentially driving the common electrodes of panel 19, and timing controller 18 for supplying a variety of timing signals to A/D converters 11, 13, and 15 and drivers 12, 14, 16, and 17. In panel 19, shown in FIG. 4, the segment electrodes of R (red) and G (green) are extracted from one side, e.g., the upper side of the panel, and the segment electrodes of B (blue) are extracted from the lower side thereof. Note that FIG. 3 shows a case wherein panel 19 consisting of 112.times.144.times.3 dots, as shown in FIG. 4, is to be driven.
In the above arrangement, signal R of red is A/D-converted into, e.g., four bit data DR1 through DR4, by A/D converter 11 and supplied to driver 12. Driver 12 outputs data of one scanning line, as a liquid crystal drive signal, to segment electrodes R1 through R144 of panel 19. Similarly, signals G and B of green and blue are A/D-converted by A/D converters 13 and 15, and then output from drivers 14 and 16 to segment electrodes G1 through G144 and B1 through B144 of panel 19, respectively. In this case, three pairs of A/D converters 11, 13, and 15 and drivers 12, 14, and 16 are controlled to operate at the same timing by controller 18. Common electrodes C1 through C112 of panel 19 are sequentially driven by driver 17. By the above series of operations, a color image is displayed on panel 19.
According to the above conventional method, since three A/D converters are required, this increases the manufacturing cost and power consumption of the apparatus. Moreover, since two arrays of segment electrodes (electrodes of R and G in FIG. 4) are extracted from panel 19 in the same direction, these electrodes are connected to the segment drivers in a staggered manner, resulting in poor productivity.